Magnetic media are widely used in various applications, particularly in the computer industry, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. Conventional thin-film type magnetic recording media, wherein a fine-grained polycrystalline magnetic alloy serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the magnetic material of the recording layer. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
Efficient, high bit density recording utilizing a perpendicular magnetic medium requires interposition of a relatively thick (i.e., as compared to the magnetic recording layer), magnetically “soft” underlayer or “keeper” layer, i.e., a magnetic layer having a relatively low coercivity below about 1 kOe, between a non-magnetic substrate, and a “hard” magnetic recording layer having relatively high coercivity of several kOe, typically about 3–6 kOe. The magnetically soft underlayer (e.g., of a NiFe alloy such as Permalloy) serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer, typically comprised of a Co-based alloy material, such as CoCr. In addition, the magnetically soft underlayer reduces susceptibility of the medium to thermally-activated magnetization reversal by reducing the demagnetizing fields which lower the energy barrier that maintains the current state of magnetization.
A typical conventional perpendicular recording system 10 utilizing a vertically oriented magnetic medium 1 with a relatively thick soft magnetic underlayer, a relatively thin hard magnetic recording layer, and a single-pole head, is illustrated in FIG. 1, wherein reference numerals 2, 3, 4, and 5, respectively, indicate a non-magnetic substrate, a soft magnetic underlayer, at least one non-magnetic interlayer, and a perpendicular hard magnetic recording layer. Reference numerals 7 and 8, respectively, indicate the single and auxiliary poles of a single-pole magnetic transducer head 6. The relatively thin interlayer 4 (also referred to as an “intermediate” layer), comprised of one or more layers of non-magnetic materials, serves to (1) prevent magnetic interaction between the soft underlayer 3 and the hard recording layer 5 and (2) promote desired microstructural and magnetic properties of the hard recording layer.
As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φ is seen as emanating from single pole 7 of single-pole magnetic transducer head 6, entering and passing through vertically oriented, hard magnetic recording layer 5 in the region below single pole 7, entering and travelling within soft magnetic underlayer 3 for a distance, and then exiting therefrom and passing through the perpendicular hard magnetic recording layer 5 in the region below auxiliary pole 8 of single-pole magnetic transducer head 6. The direction of movement of perpendicular magnetic medium 1 past transducer head 6 is indicated in the figure by the arrow above medium 1.
With continued reference to FIG. 1, vertical lines 9 indicate grain boundaries of polycrystalline (i.e., granular) layers 4 and 5 of the layer stack constituting medium 1. Since magnetically hard main recording layer 5 is epitaxially formed on interlayer 4, the grain boundaries 9, hence the grains of each polycrystalline layer generally are of substantially the same width (as measured in a horizontal direction) and in vertical registry (i.e., vertically “correlated” or “aligned”). Thus, overlying polycrystalline layer 5 typically replicates the grain width of underlying layer 4 and is vertically aligned therewith. Completing the layer stack is a protective overcoat layer 11, such as of a diamond-like carbon (DLC) formed over hard magnetic layer 5, and a lubricant topcoat layer 12, such as of a perfluoropolyethylene material, formed over the protective overcoat layer.
Substrate 2 is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having an Ni—P plating layer on the deposition surface thereof, or substrate 2 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials; underlayer 3 is typically comprised of an about 500 to about 4,000 Å thick layer of a soft magnetic material selected from the group consisting of Ni, NiFe (Permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFe, Fe, FeN, FeSiAl, FeSiAlN, FeCoC, etc.; interlayer 4 typically comprises an up to about 300 Å thick layer of a non-magnetic material, such as TiCr; and hard magnetic layer 5 is typically comprised of an about 100 to about 250 Å thick layer of a Co-based alloy including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, B, and Pd, iron nitrides or oxides, or a (CoX/Pd or Pt)n multilayer magnetic superlattice structure, where n is an integer from about 10 to about 25, each of the alternating, thin layers of Co-based magnetic alloy is from about 2 to about 3.5 Å thick, X is an element selected from the group consisting of Cr, Ta, B, Mo, Pt, W, and Fe, and each of the alternating thin, non-magnetic layers of Pd or Pt is about 1 Å thick. Each type of hard magnetic recording layer material has perpendicular anisotropy arising from magneto-crystalline anisotropy (1st type) and/or interfacial anisotropy (2nd type).
An improvement in the signal-to-media noise ratio (SMNR) and thermal stability of fine-grained, high areal recording density perpendicular magnetic recording media, such as illustrated in FIG. 1, is obtained by increasing the number of magnetic particles (grains)/unit area of the media, e.g., by decreasing the effective grain size. One approach for obtaining media with decreased effective grain size involves formation of laminated media (hereinafter designated “LM”), comprising two or more pairs of stacked magnetic layers wherein adjacent magnetic layers are spaced apart by a non-magnetic spacer layer, the non-magnetic spacer layers being thicker than those utilized in anti-ferromagnetically coupled (hereinafter designated “AFC”) media. That is, in LM the spacer layer is provided for physically spacing and magnetically coupling rather than magnetically coupling the layers, and, as a consequence, the strength of any magnetic exchange coupling between the stacked ferromagnetic layers is much smaller than the magnetic energies of the grains of each layer.
Referring to FIG. 2, illustrated therein, in schematic, cross-sectional view, is a portion of a conventional perpendicular LM 20 having a structure generally similar to media 1 of FIG. 1, but wherein the layer stack has been modified to include a pair of laminated polycrystalline, perpendicular magnetic layers, comprising in overlying sequence from the first (or lower) polycrystalline perpendicular hard magnetic layer 5, a polycrystalline, non-magnetic spacer layer 13, e.g., an about 1 to about 20 nm thick layer of Cr, Ti, or a non-magnetic CoCr alloy, and a second (or upper) polycrystalline perpendicular hard magnetic layer 5′. As illustrated, the grain boundaries of the polycrystalline layers 4 and 5 designated by reference numeral 9 extend substantially vertically into the overlying polycrystalline, non-magnetic spacer layer 13 and the second (or upper) polycrystalline perpendicular hard magnetic layer 5′, thereby indicating the substantially vertical correlation (i.e., vertical registry or alignment) and substantially same width of the grains of each overlying polycrystalline layer of the layer stack, including the laminated pair of polycrystalline, perpendicular magnetic layers 5 and 5′.
As indicated supra, in such conventional LM the laminated polycrystalline magnetic layers are spaced apart by means of polycrystalline, hetero-epitaxial non-magnetic spacer layers 13, such that the grains of each overlying polycrystalline layer, e.g., (1) a first (or lower) polycrystalline magnetic layer, (2) a polycrystalline, non-magnetic spacer layer, and (3) a second (or upper) polycrystalline, perpendicular magnetic layer, are of substantially the same width and substantially vertically correlated (i.e., in vertical registry or alignment). Stated differently, the crystallographic orientation and small grain size are maintained in each of the polycrystalline, perpendicular magnetic layers 5 and 5′. Unfortunately, however, the vertical correlation or registry of the grains of each of the polycrystalline, perpendicular magnetic layers induces magnetostatic coupling therebetween, wherein vertically correlated or aligned pairs of grains act as a single magnetic switching unit. As a consequence, the effective grain size is not reduced to the extent possible if the grains were not spatially, i.e., vertically, correlated.
According to another approach in the conventional art for improving SMNR, etc., of magnetic recording media, again involving formation of laminated media, the non-magnetic spacer layer(s) 13 does (do) not maintain the epitaxial growth orientation of the first (or lower) polycrystalline magnetic layer 5, and thus the grains of the second (or upper) polycrystalline magnetic layer 5′ do not preserve the epitaxial growth orientation, hence vertical correlation or alignment with the first (or lower) polycrystalline magnetic layer 5. Thus, while in this instance, the positions of the grains in the second (or upper) polycrystalline magnetic layer 5′ may be vertically uncorrelated (i.e., unaligned) with the grains of the first (or lower) polycrystalline magnetic layer 5, the optimized microstructure and crystallographic orientation of the first (or lower) polycrystalline magnetic layer 5 afforded by the underlying interlayer 4 are effectively absent in the second (or upper) polycrystalline magnetic layer 5′, whereby the full benefit in grain size reduction obtainable by lamination with the first (or lower) polycrystalline magnetic layer 5 is lost.
In view of the foregoing, there exists a clear need for laminated perpendicular magnetic recording media (LM), wherein both the upper and lower polycrystalline, perpendicular magnetic layers are of optimal microstructure, such that the grains of each layer are crystallographically well oriented and positionally (i.e., vertically) uncorrelated with the grains of any other magnetic layer.
The present invention, therefore, addresses and solves problems attendant upon the design and manufacture of improved structures for perpendicular laminated media, which structures comprise at least a pair of laminated polycrystalline perpendicular magnetic layers separated by a polycrystalline, non-magnetic spacer layer.